Known gasifiers make it possible to produce a fuel gas from a solid carbon-based fuel, in particular from wood waste, such as that originating, for example, from saw mills or from forestry, or from agricultural byproducts (straw, and the like), or also from recycled wood. This fuel gas comprises in particular carbon monoxide and hydrogen and can subsequently be used for various purposes, such as, for example, to feed a gas turbine or an internal combustion engine, a boiler, or a furnace.
However, the majority of known cocurrent gasifiers provide a gas also comprising a not insignificant amount of tars, which can harm the satisfactory operation of the equipment in which such a gas is used as fuel. Various solutions have thus been provided in order to reduce the content of tars of the gas produced by such gasifiers.
European Patent EP 1 248 828 discloses, for example, a gasifier in which an empty space (that is to say, a region devoid of solid material) is created in the combustion region in order to obtain better combustion of the pyrolysis gases and also better gasification of the pyrolyzed mass, which makes it possible to reduce the content of tars of the gas at the outlet. In order to create this empty space, this patent proposes to equip the lower part of the reduction region with a mechanism that makes it possible to regulate the transfer of solid material between the reduction region and the region for collecting the ashes.
The lower part of the pyrolysis region is furthermore equipped with funnels and with a movable grid in order to more or less meter out the amount of solid fuel entering the combustion region.
Such a system exhibits the disadvantage that, in view of the highly random nature of the flows of solids, it is possible for material not yet completely pyrolyzed to enter the combustion region. Furthermore, it might also possibly be that material not yet completely reduced enters the region for collecting the ashes. This is because, in the case where the flow rate of material entering the combustion region is faster than anticipated, the means for transferring material to the region for collecting the ashes will open to a greater extent in order to maintain the empty space in the combustion region. In fact, this incoming flow rate can vary according to circumstances, for example as a function of the physical characteristics of the biomass used (for example particle size measurement) and/or of the momentary characteristics of the flow.
Dutch Patent NL-8200417 discloses a similar gasifier and proposes to equip the lower part of the pyrolysis region with a mechanism which makes it possible to transfer solid material from the pyrolysis region to the reduction region while leaving an empty space between these two regions. This solid material transfer mechanism comprises a cone placed at a distance from a corresponding conical narrowing of the vessel and which can be rotated and/or axially moved in order to stir the solid material in order thus to transfer it to the reduction region. Here again, in the light of the highly random nature of the flows of the solids, it is possible for fuel which has not yet been completely pyrolyzed to enter the combustion region. As for the preceding example, “chimney” and/or “avalanche” phenomena (reference was made to the nature of the solid flow) can, for example, appear in the pyrolysis region. If appropriate, solid material freshly introduced into the vessel (and thus not yet completely pyrolyzed) might be carried to the reduction region by the transfer mechanism, which will bring about an increase in the content of tars of the gas at the outlet.
Although very different in their structure and their operation, there also exist countercurrent gasifiers, such as that described in the International Patent Publication WO 2008/107727 and which also involves a movable grid in order more or less to meter out the solid material entering the reduction region. Such a movable grid exhibits the same disadvantages as those described above.